Blade tip speed??

[gogetter]

I use a 21" Craftsman push mower for some real small areas that the exmark walk behind won't fit into.
The cut isn't great. It's a new mower. Blades have been sharpened. It's got a 6.75hp motor. However, it doesn't sound like it's running high enough. So I'm thinking that the blade tip speed isn't high enough causing the crappy cut.

Just kinda wondering if anyone knows what the average blade tip speed is for a 21" craftsman as compared to a commercial type 21" like a Toro, Exmark, etc.??

Thanks.

[awm]

they come muffled dn a lot more these days plus goveners are set at a low max rpm. combine that with 101/2 inch length on each side of spindle and i think you could be right . low blade speed. i think its against the law to change the settings .
but it can be done.

[65hoss]

Speed is part of the problem. These mowers are not known for the best cut.

[khouse]

I'm not for sure but I think the maximum tip speed is 19,200 feet per minute. So a small blade can spin faster than a large blade. I don't know why it's 19200 because a 21 inch blade turning 3600 rpm has a tip speed of 19845 fpm. It would be safe to set you high idle speed to 3600 rpm.

[EJK2352]

Check out the blade on your Craftsman & compare it to the ones found on a Toro, Lawn-Boy, Honda, etc. The blade on those name brand mowers is alot stiffer so it doesn't flex and will give a cleaner cut. Your mower is probably running a little on the slow side also. I can't remember the max engine speed for a 21" mower engine. You might want to check with a reputable dealer on this, as they publish it in the Briggs & Stratton service manual.:) ;) :) ED

[gogetter]

Thanks for the replies so far guys.

Ya, I know the Craftsman isn't meant for the best cut, but it doesn't get used much and I couldn't justify spending $700 to $1,000 for a commercial unit.

I did a search about raising the rpm by adjusting the governer, but seems that this greatly reduces engine life. But then again, seeing as I use it very little, for very short periods, maybe it would be worth it anyway.

I'll look into a better blade too. Thanks again.

[khouse]

Most small gas engines are rated at 3600 rpm. Yours is no different. Have a shop check and raise the rpm's to 3600. There won't be any problems with tip speed. Yes, raising the rpm's will shorten the life of an engine. But a slower running, lugging engine will overheat and it too will have a shorter life. 3600 rpm is where it should be running.

[SLS]

Hey gogetter,

I just looked in my Briggs & Stratton single cylinder 4-cycle repair manual.

The table provided in the 'governors' section shows as follows:

18" @ 3800 rpm (No Load)
19" @ 3600 rpm //
20" @ 3400 rpm //
21" @ 3250 rpm //
22" @ 3100 rpm //
23" @ 2950 rpm //
24" @ 2800 rpm //
25" @ 2700 rpm //

Idle speed 1750 for aluminum engines.
// // 1200 for cast-iron engines.


The manual also states:

'Run engine at half throttle to allow the engine to reach normal
operating temperature before measuring speed with tachometer. To account for tolerances, which may be required by tachometer manufacturers, we suggest taht the top governed speed of the engine be adjusted at least 200 RPM lower than the maximum speeds shown."

I hope this helps.

[awm]

according to mr exmark on that forum .longer blade means higher bladetip speed ,unless i misunderstood what he was saying.
remember we talkin blade tip speed.
i assumed that meant that say a 60 cut would have higher
bladetip than my 52. assuming rpm the same. now ill
mabe learn something i didnt know. ill tune in tomorrow
later now

[65hoss]

That was the way I understood it also AWM.

[SLS]

The table from the Briggs & Stratton manual above give the blade tip speed as 19,000 FPS (across the board).

So it would make sense that the longer the blade the faster the tip speed for a given engine rpm.

Wind up the 21" blade to 3800 rpm (instead of 3250) and it would be spinning FASTER than the 19,000 fps given...thus covering more inches per second than the 18" blade would (still at 19,000).

If you look at it in terms of tires, a larger circumferance will cover more distance than a smaller tire would if they were run at the same RPM's. The smaller tire would be spinning faster (rpm's) than the larger tire but the larger tire would be covering more ground (fps's).

Exmark dude is correct...and you guys understood him correctly! :)

[khouse]

Blade tip speed is defined as FPM (feet per minute) not RPM. So a longer blade will travel more feet than a shorter blade at the same RPM. Here's my formula. I think it's right. Blade length in feet X's 3.15 X RPM's ='s FPM

21in blade divided by 12in ='s 1.75 feet x's 3.15 =5.5125 circumferense in feet x's 3600 RPM's ='s 19,845 Feet per minute.

Forgive the spelling.

[khouse]

If you want to get the 21 inch blade to turn 19,200 fpm then the engine rpm would be 3483. But I still would run it at 3600 rpm. Just my preference

[lawnMaster5000]

awm, you cant say anything about corelation between RPM and blade FPM on any mower that does not have a direct drive blade. The pulleys and such will change the ratio from 1:1 to an unknows. unless of course you know this. Also, a 36 has bigger blades than a 48", but in most cases the same size blades as the 52"

so now what are you thinking? confusing?

[awm]

yup but not to worry. still remember that rithmatic teacher
talkin about pie an stuff.waited all day didnt ever see no pie.:D
these formulas over my head . im still at the 4 apples take away 3 what you got left. exception is counting money

[65hoss]

Yep, I fight anyone trying to take away my apples. :blob2:

[geogunn]

I think the old 3.5 horse engines were stronger than the new 6 plus hp engines.

GEO